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Conceptually, you are spot on. Operationally, it's tricky. The best expert in what the job requires would be the manager and the future co-workers. Unfortunately, those folks have full-time jobs to attend to***, and even if there is enough of a lull to be involved in selecting, aren't well-versed in what makes for good selection procedures. "I'll know it when I see it" is not the most dependable of methods. Of course, turning it over to full-time HR professionals who understand selection, but maybe much less about THAT job in particular, brings its own cluster of challenges.

The best would be HR professionals who are blessed with the time and acumen to ask the right questions of the manager and coworkers, and levels of management above that, to identify what the ideal incumbent needs to be able to do, now, and in the future. And I imagine you can figure out what the odds of that happening are.

(***The received wisdom is that 20% of the managers do 80% of the staffing across the board. Some fill a position every couple of years, simply because of the size of their unit and low turnover of folks in that sort of work. Others will tell us it seems like they never get off the staffing treadmill, constantly backfilling positions in their unit as folks move on. Line managers will do more staffing than, say, folks in a policy shop.)